Polyurethanes which are prepared by reacting a polyol with a polyisocyanate discolor when cured at elevated temperatures. This is caused by impurities in the polyisocyanate which are not easily separable by distillation.
Recently, there has developed a market for colorless products made of polyurethane. However, methods to provide such a product to the marketplace have resulted in changes in physical properties of the product. This has limited the use of colorless polyurethanes to products having lower mechanical property requirements, for example, compression and strength characteristics for a chair cushion versus a windsurfer board.
Prior art methods to stabilize the color of polyurethanes have been developed. The approach of these methods is either purification or stabilization of the polyisocyanate. For example, U.S. Pat. No. 4,297,472 teaches adding a small amount of vicinal diols to the formulation to improve the color stability of the polyurethanes. U.S. Pat. No. 4,065,362 teaches a process to purify the organic isocyanate, U.S. Pat. No. 3,715,381 teaches incorporating 2,4-di(t-butyl)-p-cresol (BHT) in the formulation, and U.S. Pat. No. 2,957,903 teaches incorporating an amount of triarylphosphite in a polyisocyanate. However, discoloration of the final product still occurs. Reasons for the discoloration include impurities in the starting materials, temperature effects and the processing procedure itself, as well as breakdown of the product by light and/or temperature effects over a period of time. This problem is most pronounced in thermoplastic polyurethanes prepared from aromatic isocyanates, for example, methylene diphenyl diisocyanate (MDI).
This invention solves this problem by incorporating an additive composition designed specifically to stabilize the formulation and thus the color of the final product.